The present invention relates to a power amplifier having a negative feedback circuit for a transmitter and more particularly to a negative feedback circuit for compensating for a nonlinear distortion used in the transmitter and a method for controlling the phase of the circuit in the power amplifier.
A radio system arranged to use a linear digital modulation system such as 16QAM (Quadrature Amplitude Modulation) or xcfx80/4 shift QPSK (Quadrature Phase Shift Keying) essentially needs nonlinear distortion compensation for a power amplifier. In actual, the radio system utilizes various kinds of nonlinear distortion compensating systems (linearizers). Of those systems, a Cartesian loop negative feedback type linearizer has been conventionally used. The conventional linear feedback amplifier will be described with reference to FIG. 2. FIG. 2 is a block diagram showing an arrangement of a transmitting section of a digital radio transmitter provided with the Cartesian loop negative feedback type linenarizer.
A numeral 1 denotes a baseband signal generator, which operates to output an in-phase component (called an I-component) and a quadrature component (called a Q-component) of a baseband signal. The I-component is added to the feedback signal in an adder 2-1 and then the added signal is outputted and is applied into a loop filter 3-1. Likewise, the Q-component is added to the feedback signal in the adder 2-2 and then the added signal is applied into a loop filter 3-2. The loop filters 3-1 and 3-2 operate to restrict the bandwidths of the I-component and the Q-component and then apply the resulting components into a quadrature modulator 4.
A numeral 11 denotes a reference signal generator, which operates to generate a reference frequency signal and then apply the reference signal into PLL frequency synthesizers 12 and 13. The PLL frequency synthesizer 12 operates to generate a first local oscillating signal (called an LO signal) on the basis of the reference signal and then apply a first LO signal into a quadrature modulator 4 and a phase shifter 18. The PLL frequency synthesizer 13 operates to generate a second LO signal on the basis of the reference signal and then apply the second LO signal into mixers 6 and 15. The phase shifter 18 operates to control the phase of the first LO signal through the use of a control signal to be applied from a phase controller 19 and then apply the first LO signal whose phase is controlled into the quadrature demodulator 16.
The quadrature modulator 4 operates to orthogonally modulate the first LO signal (carrier) into a signal of an intermediate frequency band (called an IF frequency band) by the I-component Ixe2x80x2 and the Q-component Qxe2x80x2 of the baseband signal to be applied therein. Then, the modulated signal is applied into a bandpass filter (BPF) 5. The bandpass filter 5 operates to remove unnecessary components from the modified signal and then apply the resulting signal into the mixer 6. The mixer 6 operates to convert the signal applied therein into a desired frequency through the use of the second LO signal outputted from the PLL frequency synthesizer 13 and then apply the converted signal into a bandpass filter (BPF) 7. The bandpass filter 7 operates to remove unnecessary spurious components from the applied signal and then send the resulting signal into the power amplifier (PA) 8. The power amplifier 8 operates to amplify the input signal up to a specified output level and then transmit the amplified signal through an antenna 9.
This negative feedback amplifier is arranged as a negative feedback linearizer based on the Cartesian loop. Hence, part of the output signal of the power amplifier 8 is fed back through a directivity coupler 10 and then is given to an attenuator (ATT) 14. The attenuator 14 operates to adjust the power level of the input signal into a proper value and then give it to the mixer 15. The mixer 15 operates to convert the signal applied from the attenuator 14 into the IF frequency through the use of the second LO signal and then apply the converted signal into the quadrature demodulator 16.
The quadrature demodulator 16 operates to produce respectively the baseband signals i and q of the I- and the Q-components by orthogonally demodulate the converted signal using the first LO signal applied from the phase shifter 18. The I-component is applied as the I-component i of the feedback signal into a subtracting input side of the adder 2-1 through a switch 20-1, while the Q-component is applied as the Q-component q of the feedback signal into the subtracting input side of the adder 2-2 through a switch 20-2. At this time, the output sides of the switches 20-1 and 20-2 are connected to the adders 2-1 and 2-2, respectively.
In this type of negative feedback, for stabilizing the system, it is necessary to keep the input signals I and Q the same as the feedback signals i and q in phase (no phase difference) on the input sides of the adders 2-1 and 2-2. That is, if the phase difference takes place between the input signal and the feedback signal, it is necessary to make the phase difference zero by controlling the phase to be shifted by a radian at the maximum.
In turn, the method for controlling the phase will be described below. At first, the switches 20-1 and 20-2 shown in FIG. 2 are switched to be connected to the phase controller 19 so that the feedback loop is held in an open state.
The baseband signal generator 1 operates to apply a predetermined DC voltage into only the I-component for the purpose of adjusting the phase. The Q-component is kept zero (Q=0). In this state, the quadrature modulation is proceeded along the foregoing operation and then the signal is sent out through the antenna 9. Then the output waveform of the power amplifier 8 takes a non-modulated carrier. The output of the power amplifier 8 is partially fed back by the directivity coupler 10. Consider the output of the feedback signal of the quadrature demodulator 16 along the foregoing operation. If the phases coincide with each other, the DC voltage appears only on the I-component side, while no signal (DC) appears only on the Q-component side. However, if the phases do not coincide with each other, the DC voltage corresponding to the phase shift appears on the output of the Q-component side. Hence, the angle of rotation of the phase can be derived from the DC voltages of the I and the Q-components.
In the phase controller 19, the phase corresponding to the derived angle of rotation is reversely controlled by controlling the phase shifter 18 so that the phase of the first LO signal may be adjusted. By coinciding the output of the feedback signal of the quadrature demodulator 16 with the input signal, the negative feedback may be stabilized. Since the phase fit of the input signal and the feedback signal makes the output on the Q-component side zero, then the switches 20-1 and 20-2 are changed to the adders 2-1 and 2-2, respectively, so that the loop is made closed.
In the foregoing prior art, each time the phase is adjusted, it is necessary to open and close the feedback loop. It means that the loop is open while the phase is being adjusted. Hence, the phase adjustment cannot be executed for the change of phase while the transmission is in operation (closed loop). Further, the switching means for opening and closing the loop is located so that the phase may be controlled by the DC voltage of the feedback signal on the input side of the switching means. Hence, the voltage drop in the switching means in the open loop is different from that in the closed loop, so that the compensation of the offset voltage of the system set in the closed loop is not adaptive to the open loop. It means that the precise phase control cannot be executed. Further, if the phase is changed by changing the phase characteristic because of the temperature variation and the gain variation, the phase variation cannot be detected.
If the operation is continued as the phase is being shifted from the fitted phase, the overall system is made to have no phase allowance and in the worst case the oscillating phenomenon may take place. In the process, the spurious signal may be generated so that the output operation characteristic may be degraded.
In U.S. Pat. No. 5,066,923 issued on Nov. 19, 1991 to Gailus et al., a method has been disclosed for switching the feedback loop as shown in FIG. 2 into the open loop, for adjusting the phase. In the specification disclosed in the Japanese Patent No. 2746133 assigned to Nippon Electric, Ltd., the invention has been disclosed for adjusting the phase as keeping the closed loop. The latter invention, however, essentially needs both of the I-component and the Q-component as the feedback signal for the purpose of detecting the phase difference and compares the input baseband I-component with the feedback I-component in phase and the input baseband Q-component with the feedback Q-component in phase. The invention hence needs a complicated circuit.
It is an object of the present invention to provide a power amplifier having a feedback circuit of a transmitter which is arranged to control the phase for constantly keeping the output operation characteristic stable even while the feedback loop is operating in the closed state and a method for controlling the phase of the feedback circuit.
The feedback circuit of a transmitter and the method for controlling the phase of the feedback circuit according to the invention are arranged as follows. An adder operates to add an I-component baseband input signal to an I-component baseband feedback signal to produce an I-component adding signal and add a Q-component baseband input signal to a Q-component baseband feedback signal to produce a Q-component adding signal. An oscillator operates to generate a carrier signal. A modulator performs quadrature modulation with respect to the carrier signal through the use of the I-component adding signal and the Q-component adding signal. An amplifier circuit operates to amplify the quadrature-modulated signal. A demodulator performs orthogonal demodulation with respect to the part of the output of the amplifier circuit by the carrier signal and then outputs the I-component baseband feedback signal and the Q-component baseband feedback signal. Then, the phase of a selected one of the I-component baseband input signal and the Q-component baseband input signal is compared with the phase of the adding signal of the same component as that of the selected component baseband signal, for the purpose of detecting if a phase shift exists. If the phase shift exists, the phase of the carrier signal is adjusted so that the phase shift may be reduced.